Several months ago I became the plant manager of a production facility that takes rolled steel and performs stamping, welding, drilling, vibratory finishing, zinc phosphating, and electrocoating in order to produce our customers' products. The wastewater from our vibratory finishing, zinc phosphating, and electrocoat operations are pretreated before discharge and regulated under the Metal Finishing pretreatment standards by our local sewer district. Our owners insist, and I agree, that we need to keep the factory floor clean. Therefore, every day one of the maintenance people spends 1-2 hr cleaning a portion of the plant with a powered floor scrubbing machine. Recently, I observed him emptying the machine into the janitor's closet drain and asked him how long that has been the plant's practice; he responded, "many years." After further investigation, I determined that this drain is connected directly to the sanitary sewer. Is this wastewater regulated? Should it go to wastewater pretreatment system? C.B.
You have asked a question about a common issue in many manufacturing plants. "Scrubber water" or "mop water" is always a very oily, dirty waste that people just as soon get rid of into the nearest drain, and because it is associated with plant housekeeping activities, plant personnel can view it as "sanitary" waste. However, EPA's final "Development Document for Effluent Limitations Guidelines and Standards for the Metal Finishing Point Source Category" states that "water . . . used for washing away floor spills . . . washing equipment such as filters, pumps, and tanks, picks up residues of concentrated process solutions, salts, or oils" are process waste streams and are to be "routed . . . for treatment." Based upon further discussions with my regulatory contacts, it was confirmed that "floor or mop water" generated in production or process areas is regulated under the Metal Finishing pretreatment standards; however, "floor or mop water" generated in warehouse, employee welfare, or offices is not regulated under Metal Finishing pretreatment standards but would be regulated under the sewer district's local limits. The most likely troublesome local limit to meet is oil and grease with local limits in the 50-200 mg/liter (ppm) range, while the "floor or mop water" may contain oil and grease in the 1,000's ppm range.
You have three options: route the waste to your wastewater pretreatment system, concentrate the oil by evaporation, or collect it for off-site disposal. Treating this waste stream on-site may not be as easy as one may think. The very high concentrations of oils as well as the detergents and cleaners in this waste can wreak havoc on your wastewater pretreatment system. Unless the volume of "floor and mop water" is extremely small compared to your process wastewater flow (very much less than 1%), and you are able to slowly bleed this waste into your system, you will need to pretreat this waste stream. There are three general pretreatment schemes. I would consider the following: oil breaking, coagulation, and membrane filtration.
You can "break" out the oil by lowering the pH to the range of 3-5 and then allow the free oil to float to the surface and be skimmed off. However, this does not remove the detergents or cleaners, although the acidic condition may be sufficient to destabilize them so that they do not significantly interfere with the main pretreatment system.
To coagulate the oils, you can use inorganic coagulants such as polyaluminum chloride, aluminum sulfate, or ferric chloride and/or cationic or nonionic polymers, or you can use one-step chemically treated organoclays, which are "engineered" bentonite clays. In this scheme, the oil is captured by the coagulants to form a floc particle that either floats or settles. If the floc floats, you will need to skim it off and send it directly to your filter press and then send the remaining wastewater to pretreatment system; if the floc settles, you can send the wastestream to your pretreatment system's floc tank. These coagulants will likely not remove any cleaners or detergents but should be able to destabilize them so that they do not interfere with the main pretreatment system. I recommend that you discuss the above two options with your wastewater chemical supplier who should be able to perform jar tests to determine the most effective pretreatment as well as estimate chemical costs.
Membrane filtration is also a good candidate for pretreatment. Here you would recirculate the contents of the waste holding tank through a particle filter and then an ultrafilter in order to concentrate the oil and some of the cleaners and detergents in the waste holding tank. The "clean" membrane filtrate would then be routed to the main pretreatment system, although you may need to modify your pretreatment chemistry to handle the detergents that could still remain in the filtrate. When the oil concentration in the holding tank rises to a sufficiently high level, you can send it off-site as waste oil.
By the use of an atmospheric evaporator, the wastestream is heated in order to drive off the water and concentrate the dirt and oil. When the oil concentration in the evaporator rises to a sufficiently high level, you can send it off-site as waste oil, or if it is too dirty, send it to off-site disposal as an oily waste. The value of using an atmospheric evaporator in order to concentrate wastes is driven by the cost of off-site disposal as compared to the cost of evaporation.
Suppliers of chemicals and membrane filtration systems for the treatment of oily wastes can be found in the Products Finishing 2002 Directory and Technology Guide under "flocculants, polyelectrolytes", "oil skimmers, separators," "pollution control equipment, water: evaporative recovery," and "pollution control equipment, water: membrane filtration."
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